MEMS technology has been under steady development for some time, and as a result various MEMS devices have been considered and demonstrated for several applications. MEMS technology is an attractive approach for providing inertial sensors, such as accelerometers for measuring linear acceleration and gyroscopes for measuring angular velocity. A MEMS inertial sensor typically includes a proof mass which is flexibly attached to the rest of the device. Relative motion between the proof mass and the rest of the device is driven by actuators and/or sensed by sensors in various ways, depending on the detailed device design. Other MEMS applications include optical applications such as movable mirrors, and RF applications such as RF switches and resonators.
Since MEMS fabrication technology is typically based on processing planar silicon wafers, it is useful to classify MEMS devices according to whether the actuation and/or sensing performed in an inertial sensor (or other application) is in-plane or out of plane (i.e., vertical). More specifically, a device is “in-plane” if all of its sensing and/or actuation is in-plane, otherwise it is “vertical”. In-plane devices tend to be easier to fabricate than vertical devices, but vertical devices tend to provide performance advantages compared to in-plane devices. Furthermore, certain device functions, such as sensing rotation about an in-plane axis, cannot be performed by in-plane devices. Thus vertical MEMS devices are also undergoing steady development, despite fabrication difficulties that tend to increase cost.
One approach which has been used to fabricate vertical MEMS devices is hybrid integration, where elements of a MEMS assembly are individually assembled to form the desired vertical structure. For example, attachment of a spacer to a substrate, followed by attachment of a deformable diaphragm to the spacer, provides a vertical MEMS structure having a spacing between diaphragm and substrate controlled by the spacer. U.S. Pat. No. 6,426,687 provides further information on this approach. Although hybrid integration can provide vertical MEMS devices, the cost tends to be high, since manual processing steps are usually required, and because hybrid integration is typically performed on single devices. Therefore, there is a need for reduced cost integrated MEMS devices that is unmet in the prior art.